Water-glycol hydraulic fluid
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a water-glycol hydraulic fluid, or more particularly to a water-glycol hydraulic fluid having high wear resistance and excellent sludge solubility. 
     2. Description of the Prior Arts 
     In general, water-glycol hydraulic fluid is widely used as hydraulic fluid for various kinds of equipment such as hydraulic instruments since it is fire resistant or flame retardance. But it has a problem such that its wear resistance is unsatisfactory. It is known that an aliphatic acid can be mixed to the water-glycol hydraulic fluid in order to solve the problem. For example, Japanese Patent Publication No. 59159/1987 discloses that a higher unsaturated aliphatic acid and a higher saturated aliphatic acid are used in combination, and alkali hydroxide in an amount of more than the neutralization equivalent amount is mixed therein to prepare a water-glycol hydraulic fluid with improved wear resistance. 
     However, in the water-glycol hydraulic fluid with improved wear resistance by addition of a higher unsaturated aliphatic acid, the higher unsaturated aliphatic acid is liable to be deteriorated, and as a result, sludge solubility of the hydraulic fluid is lowered to precipitate a solid material dissolved therein in some cases. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a water-glycol hydraulic fluid with improved wear resistance. Another object of the present invention is to provide a water-glycol hydraulic fluid with excellent sludge solubility as well as improved wear resistance. 
     The present invention provides a water-glycol hydraulic fluid which comprises a higher aliphatic acid in the ratio of 2 to 15% by weight, alkali hydroxide in an amount less than an equivalent amount of said higher aliphatic acid and an organic alkaline compound containing nitrogen atoms in an amount sufficient to adjust the pH to 10 to 12 in the water-glycol hydraulic fluid containing water and glycol. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The mixing ratio of water and glycol in the water-glycol fluid of the present invention is not limited and can be properly determined depending on various conditions. For example, the ratio of water is 30 to 70% by weight and the ratio of glycol is 70 to 30% by weight. Further if desired, a viscosity adjusting agent to adjust the viscosity of the hydraulic fluid can be added thereto in an amount of 5 to 60 parts by weight based on 100 parts by weight of the total of said water and glycol. 
     As the above-described glycols, there can be cited monoethylene glycol, diethylene glycol, triethylene glycol, monopropylene glycol, dipropylene glycol, tripropylene glycol, monohexylene glycol, dihexylene glycol, trihexylene glycol, etc., and these can be used singly or in combination. In general, it is preferable to use mono- or di-propylene glycol. 
     Also, as a viscosity adjusting agent, a copolymer of ethylene oxide (EO) and propylene oxide (PO) is usually used. The average molecular weight of this copolymer is suitably about 5,000 to 20,000, and further, the ratio of EO and PO in the copolymer is preferably 10:1 to 1:10 (mol ratio). 
     As the higher aliphatic acid to be added to the water-glycol liquid prepared as described above, there can be used various kinds of those used hitherto, for example, saturated aliphatic acid, unsaturated aliphatic acid, straight chain aliphatic acid, branched chain aliphatic acid and mixture thereof, as long as they have a carbon number of about 10 to 22. Among them, it is preferable to use a saturated aliphatic acid having a carbon number of 12 to 18, when viewing the aspect of the defoaming properties and solubility. The mixing ratio of the higher aliphatic acid is suitably 2 to 15% by weight based on the whole amount of the hydraulic fluid, and preferably 5 to 12% by weight. When it is less than 2% by weight, the hydraulic fluid obtained becomes deficient in wear resistance, and when it exceeds 15% by weight, the solubility in the water-glycol liquid becomes poor and there is a fear of separating out of the higher aliphatic acid itself. 
     Examples of alkali hydroxides include hydroxides of alkali metals, or more specifically, potassium hydroxide, sodium hydroxide, lithium hydroxide, etc. The mixing ratio of this alkali hydroxide is preferably adjusted to be less than an equivalent amount of the above-described higher aliphatic acid, and especially in such a manner as the mol ratio to the higher aliphatic acid becomes 0.95 to 0.995. When the mixing ratio is too small, the wear resistance becomes deficient, and the solubility of sludge also deteriorates. It is also the same in the case where the alkali hydroxide is added in an amount more than the equivalent amount of the higher aliphatic acid. 
     In the present invention, an organic alkaline compound containing nitrogen atoms is added together with the components described above. The amount of the organic alkaline compound may be an amount sufficient to regulate the pH of the whole system to be 10 to 12. This amount sufficient for regulating the pH of the whole system to 10 to 12 is different depending on various kinds of conditions, and can not be determined unconditionally, but usually it is determined in the range of 0.5 to 5% by weight based on the whole system. 
     When the pH of the whole system is less than 10, the sludge solubility deteriorates. On the other hand, when the pH exceeds 12, the wear resistance becomes poor. As the organic alkaline compound containing nitrogen atoms to be added for adjusting the pH, various kinds can be used. Typical examples are morpholine, cyclohexylamine, dicyclohexylamine, dimethylethanolamine and diethylethanolamine. In particular, morpholine and cyclohexylamine are preferred. 
     Further, for the water-glycol hydraulic fluid of the present invention, various kinds of additives can be added, if necessary, such as, metal deactivators, defoaming agents and coloring agents. Although the amount of these additives may be approximately the same as in the conventional cases, it is suitable to be in the range of 0.1 to 0.5% by weight as the total amounts. Examples of the metal deactivators include benzotriazole, methylbenzotriazole, mercaptobenzothiazole, benzothiazole, ethylenediaminetetraacetic acid (EDTA), EDTA disodium salt and EDTA tetrasodium salt. Among them, benzotriazole is preferred. 
     As explained above, the water-glycol hydraulic fluid of the present invention has excellent wear resistance, and has also excellent sludge solubility. Further, the appearance of the fluid after using for a long period of time is good, and the amount of foaming is small. 
     Therefore, according to the water-glycol hydraulic fluid of the present invention, the required exchange period of various parts of pump main bodied and filters and the like can be prolonged, and since it has excellent sludge solubility, the working damage of the instrument due to the precipitation of solid materials can be prevented. 
     Next, the present invention will be explained in more detail by referring to Examples and Comparative Examples. 
     EXAMPLES 1 TO 9 AND COMPARATIVE EXAMPLES 1 TO 4 
     As shown in the following Table, lauric acid, oleic acid, and isostearic acid as the higher aliphatic acids, potassium hydroxide with purity of 86.0% by weight as the alkali hydroxide, morpholine as an organic alkaline compound containing nitrogen atoms, propylene glycol and dipropylene glycol as the glycol, and the above-described EO-PO copolymer (EO:PO=4:1, molecular weight of about 15,000), and further, metal deactivator, defoaming agent and coloring agent as various kinds of additives, were mixed with each other in weight percentage as shown in the Table to prepare a water-glycol hydraulic fluid. 
     Succeedingly, a pump test was carried out according to the ASTM D 2251, and the characteristics of the fluid was evaluated. A V-104C type pump was used, and the operating conditions were such that the pressure was 70 kg/cm 2 , the temperature 50° C., the rotation number 1200 r.p.m., and the operating time 250 hours. 
     Also, the evaluation as a hydraulic fluid was carried out on the basis of the total of the wearing amount of the vane and the cum ring of the pump, the generation or not of a floating substance (sludge), appearance, transparency, and foaming properties (according to JIS K2231). As to the appearance of the liquid, the one which is transparent and has no floating substances was marked as &#34;O&#34;, the one in which a little floating substance was observed as &#34;Δ&#34;, and the one in which a large amount of floating substances was observed as &#34;x&#34;. pH is shown only for the ones in which adjustment was effected with morpholine. 
     
                                           TABLE                                   
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                        KOH/aliphatic                                     
       lauric                                                             
           oleic                                                          
               isostesric                                                 
                        acid          propylene                           
                                            dipropylene                   
                                                  EO-PO                   
       acid                                                               
           acid                                                           
               acid KOH (mol ratio)   glycol                              
                                            glycol                        
                                                  copolymer               
No.    (wt %)                                                             
           (wt %)                                                         
               (wt %)                                                     
                    (wt %)                                                
                        (wt %)  morpholine                                
                                      (wt %)                              
                                            (wt %)                        
                                                  (wt %)                  
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Example 1                                                                 
       7.5 --  --   2.42                                                  
                        0.992   2.0   --    33.0  14.0                    
Example 2                                                                 
       7.5 --  --   2.40                                                  
                        0.984   2.0   --    33.0  14.0                    
Example 3                                                                 
       7.5 --  --   2.38                                                  
                        0.976   2.0   --    33.0  14.0                    
Example 4                                                                 
       7.5 --  --   2.36                                                  
                        0.978   2.0   --    33.0  14.0                    
Example 5                                                                 
       7.5 --  --   2.34                                                  
                        0.960   2.0   --    33.0  14.0                    
Example 6                                                                 
       7.5 --  --   2.38                                                  
                        0.976   2.0   31.0  --    16.0                    
Example 7                                                                 
       7.5 --  --   2.38                                                  
                        0.976   2.0   --    32.94 14.0                    
Example 8                                                                 
       --  10.6                                                           
               --   2.38                                                  
                        0.976   2.0   --    29.9  14.0                    
Example 9                                                                 
       --  --  10.7 2.38                                                  
                        0.976   2.0   --    29.8  14.0                    
Comparative                                                               
       6.3  1.2                                                           
               --   2.35                                                  
                        1.011   0.5   33.5  --    16.0                    
Example 1                                                                 
Comparative                                                               
       7.5 --  --   2.44                                                  
                        1.000   0.5   --    33.0  14.0                    
Example 2                                                                 
Comparative                                                               
       7.5 --  --   2.38                                                  
                        0.976   --    --    33.0  14.0                    
Example 3                                                                 
Comparative                                                               
       7.5 10.6                                                           
               --   2.44                                                  
                        1.000   2.0   --    29.9  14.0                    
Example 4                                                                 
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                                  wear      appearance                    
                    additive      amount                                  
                                      occurrence                          
                                            of    foaming                 
             No.    (wt %)                                                
                         water pH (mg)                                    
                                      of sludge                           
                                            the liquid                    
                                                  properties              
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             Example 1                                                    
                    0.06 remainder                                        
                               10.9                                       
                                  23  O     O     10&gt;˜0             
             Example 2                                                    
                    0.06 remainder                                        
                               10.8                                       
                                  21  O     O     10&gt;˜0             
             Example 3                                                    
                    0.06 remainder                                        
                               10.6                                       
                                  12  O     O     10&gt;˜0             
             Example 4                                                    
                    0.06 remainder                                        
                               10.5                                       
                                  15  O     O     10&gt;˜0             
             Example 5                                                    
                    0.06 remainder                                        
                               10.4                                       
                                  37  O     O     10&gt;˜0             
             Example 6                                                    
                    0.06 remainde                                         
                               10.6                                       
                                   7  O     O      30˜0             
             Example 7                                                    
                    0.12 remainder                                        
                               10.6                                       
                                   8  O     O     10&gt;˜0             
             Example 8                                                    
                    0.06 remainder                                        
                               10.6                                       
                                  18  O     O      40˜10            
             Example 9                                                    
                    0.06 remainder                                        
                               10.6                                       
                                  10  O     O     10&gt;˜0             
             Comparative                                                  
                    0.15 remainder                                        
                               10.3                                       
                                  58  X     O      30˜0             
             Example 1                                                    
             Comparative                                                  
                    0.06 remainder                                        
                               10.5                                       
                                  780 Δ                             
                                            O      20˜0             
             Example 2                                                    
             Comparative                                                  
                    0.06 remainder                                        
                               -- 280 X     O     10&gt;˜0             
             Example 3                                                    
             Comparative                                                  
                    0.06 remainder                                        
                               10.5                                       
                                  1370                                    
                                      X     O      30˜0             
             Example 4                                                    
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